Platelets initiate clot formation and have important roles in both innate and adaptive immunity (Henn et al., “CD40 Ligand on Activated Platelets Triggers an Inflammatory Reaction of Endothelial Cells,” Nature 391:591-594 (1998) and Wagner et al., “Platelets in Inflammation and Thrombosis,” Arterioscler Thromb Vasc Biol 23:2131-2137 (2003)). Loss of platelets either by their destruction in the periphery or their reduced production can occur in diseases such as immune thrombocytopenic purpura, thrombotic thrombocytopenic purpura, human immunodeficiency virus (HIV) infection, aplastic anemia, and acute respiratory distress syndrome and in about 1-5% of people receiving heparin therapy (Cines et al., “Heparin-induced Thrombocytopenia: An Autoimmune Disorder Regulated Through Dynamic Autoantigen Assembly/Disassembly,” J Clin Apher 22:31-36 (2007)). In addition, cancer chemotherapy and radiation therapy are two of the most common causes of thrombocytopenia. Currently, platelet transfusions are the “gold-standard” for treating the life-threatening complications of thrombocytopenia. However, platelet transfusions increase the risk of inflammation and disease transmission, are costly and not always readily available (Blumberg et al., “An Association of Soluble CD40 Ligand (CD154) with Adverse Reactions to Platelet Transfusions,” Transfusion 46:1813-1821 (2006); Kaufman et al., “Release of Biologically Active CD154 During Collection and Storage of Platelet Concentrates Prepared for Transfusion,” J Thromb Haemost 5:788-796 (2007)). A catastrophic event such as mass radiation exposure would leave many victims without treatment. Currently, recombinant human interleukin (IL-)11, the only clinically approved drug for treating thrombocytopenia, is used as an alternative to platelet transfusions to modestly raise platelet counts (Bhatia et al., “The Role of Interleukin-11 to Prevent Chemotherapy-induced Thrombocytopenia in Patients with Solid Tumors, Lymphoma, Acute Myeloid Leukemia and Bone Marrow Failure Syndromes,” Leuk Lymphoma 48:9-15 (2007)). Therefore, there remains a need for more efficacious and readily available treatments to increase platelet number.
Platelets are derived from megakaryocytes, which reside in the bone marrow (Patel et al., “The Biogenesis of Platelets from Megakaryocyte Proplatelets,” J Clin Invest 115:3348-3354 (2005)). During megakaryocyte maturation, the polyploid cell undergoes a complex process of cytoskeletal rearrangement, followed by proplatelet elongation, and the release of cytoplasmic fragments as circulating platelets (Italiano et al., “Blood Platelets are Assembled Principally at the Ends of Proplatelet Processes Produced by Differentiated Megakaryocytes,”J Cell Biol 147:1299-1312 (1999); Kaushansky, “Historical Review: Megakaryopoiesis and Thrombopoiesis,” Blood 111:981-986 (2008)). Proteomic studies have revealed that both megakaryocytes and platelets contain proteins of unknown function. It has been reported that the ligand-activated transcription factor, peroxisome proliferator-activated receptor gamma (PPARγ), is present in both megakaryocytes and platelets (Akbiyik et al., “Human Bone Marrow Megakaryocytes and Platelets Express PPARγ, and PPARγ Agonists Blunt Platelet Release of CD40 Ligand and Thromboxanes,” Blood 104:1361-1368 (2004)). PPARγ functions as a heterodimer with the Retinoid X Receptor (RXR) to regulate adipogenesis, glucose metabolism, and inflammation (Schoonjans et al., “The Peroxisome Proliferator Activated Receptors (PPARS) and Their Effects on Lipid Metabolism and Adipocyte Differentiation,” Biochim Biophys Acta 1302:93-109 (1996); Kliewer et al., “Convergence of 9-cis Retinoic Acid and Peroxisome Proliferator Signalling Pathways Through Heterodimer Formation of Their Receptors,” Nature 358:771-774 (1992)). It has also been shown that the PPARγ ligands rosiglitazone and 15d-PGJ2 dampen thrombin-induced human platelet activation and aggregation (Akbiyik et al., “Human Bone Marrow Megakaryocytes and Platelets Express PPARγ, and PPARγ Agonists Blunt Platelet Release of CD40 Ligand and Thromboxanes,” Blood 104:1361-1368 (2004)). Importantly, it was recently determined that PPARγ is also found in platelet microparticles released during activation (Ray et al., “Peroxisome Proliferator-activated Receptor Gamma and Retinoid X Receptor Transcription Factors are Released from Activated Human Platelets and Shed in Microparticles,” Thromb Haemost 99:86-95 (2008)). Initially, it was believed that PPARγ ligands would blunt the activity of platelets treated with PPARγ ligands by minimizing unwanted pro-inflammatory and/or prothrombotic responses by the platelets, and platelets produced by megakaryocytes treated with PPARγ ligands would likewise exhibit diminished pro-inflammatory and/or prothrombotic response (PCT Publ. WO 2005/041872 to Phipps et al.).
It would be desirable to identify classes of compounds that can be used to improve the production of platelets by megakaryocytes, and thereby afford improved therapeutic treatment of conditions that involve low platelet count. The present invention is directed to overcoming these and other deficiencies in the art.